1. Field of the Invention
This invention relates in general to electromechanical alignment and isolation, and more particularly to a method and apparatus for supporting and aligning a wafer in a microlithographic system having extreme precision.
2. Description of the Prior Art
Various support and positioning structures are well known for use in microlithographic instruments. Typically in the prior art, XY guides including a separate X guide assembly and a Y guide assembly are utilized with one guide assembly mounted on and movable with the other guide assembly. Often a separate wafer stage is mounted on top of the guide assemblies. These structures require high precision in manufacturing and many components. These structures are typically used in a wafer stepper apparatus where the alignment of an exposure field to the reticle being imaged affects the success of the circuit i.e., the yield. In a scanning exposure system, the reticle and wafer move simultaneously and scan across one another during the exposure sequence.
A related system is disclosed in copending and commonly owned U.S. patent application Ser. No. 08/221,375, filed Apr. 1, 1994, titled "Guideless Stage With Isolated Reaction Stage" invented by Martin E. Lee, now U.S. Pat. No. 5,528,118, issued Jun. 18, 1996, and copending and commonly owned U.S. patent application Ser. No. 08/266,999, filed Jun. 27, 1994, titled "Electromagnetic Alignment and Scanning Apparatus", invented by Akimitsu Ebihara now abandoned in favor of U.S. patent application Ser. No. 08,698,827, filed Aug. 16, 1996. See also U.S. Pat. No. 5,040,431 issued Aug. 20, 1991 to Sakino et al. and U.S. Pat. No. 4,667,139 issued May 19, 1987 to Hirai et al. All of the above patent disclosures are incorporated herein by reference. Many other examples of such stage structures, often called "XY stages", are known in the art.
Prior art stages typically suffer from a significant drawback in that the sensitivity of measurement accuracy of the stage position is adversely affected by temperature. The electromagnetic motors which drive the elements of the stage relative to one another are a significant heat source adversely affecting the performance of the laser interferometry typically used to determine the actual stage position.
Another disadvantage of prior art systems is that the numerous cables including electrical cables, fiber optic cables, coolant tubes, vacuum tubes and air hoses connecting to the stage from external devices impose a significant amount of drag and mechanical forces, both steady and impulsive, on the actual stage, thus degrading performance. Thus, cable drag occurs as the stage moves about pulling the cables with it, causing thereby mechanical friction and disturbances.
Additionally, prior art stages suffer from reduced performance due to the relatively high mass of the stage which typically carries the heavy electromagnetic drive motor magnets for positioning the stage in at least one axis direction. Higher mass may reduce the stage mechanical resonance frequency and thereby lower the stage performance. If the stage is made stiffer to compensate, this may add even more mass. Higher mass requires more motor power, thus undesirably more potential for heating.
Therefore, there is a significant problem in the prior art of impeded stage performance in terms of accuracy and speed caused by the relatively high weight of the stage support, the cable drag, and the heat generated by the stage movement impeding sensing accuracy in terms of position.